Filter circuit

ABSTRACT

This invention is a filter circuit provided in a radio communication module. First to third conductor patterns ( 8  to  10 ) having a length shorter than λ/4 of a passing wavelength λ and electromagnetically coupled with each other are formed as distributed line patterns parallel to each other in a dielectric board ( 2 ), and a first capacitor ( 16 ) and a second capacitor ( 17 ) add parallel capacitance to the first conductor pattern ( 8 ) and the second conductor pattern ( 9 ) having their distal ends short-circuited. The third conductor pattern ( 10 ) has its both end opened. As the first conductor pattern ( 8 ) and the second conductor pattern ( 9 ) carry out inductive operation and the third conductor pattern ( 10 ) is capacitive-coupled with these conductor patterns, resonance is made in a band lower than a frequency band prescribed by the length of the lines.

TECHNICAL FIELD

This invention relates to a filter circuit carried on a radiocommunication module or the like used in a microwave or millimeter waveband, and particularly to a filter circuit formed on a dielectric boardto shorten a conductor pattern forming a resonator pattern.

This application claims priority of Japanese Patent ApplicationNo.2001-379080, filed on Dec. 12, 2001 in Japan, the entirety of whichis incorporated by reference herein.

BACKGROUND ART

With the progress of telecommunication technology, radio communicationmodules are carried on various devices and systems such as variousmobile communication devices, ISDN (integrated service digital network)and computer devices, and enable high-speed communication of datainformation and the like. The radio communication modules are reduced insize and weight, combined, or made multifunctional. In high-frequencyapplications using microwaves and millimeter waves as carrierfrequencies, for example, in a communication device constituting a radioLAN (local area network) or the like, the radio communication modulescannot achieve the above-described specification requirements in acircuit based on a concentrated constant design in which a low-passfilter, a high-pass filter, a band-pass filter, a coupler and the likeuse chip components such as capacitors and coils, and a distributedconstant design using microstrip lines, strip lines and the like isgenerally is used.

Conventionally, a band-pass filter (BPF) 100 based on a distributedconstant design is formed by cascading plural resonator conductorpatterns 102 a to 102 e on a major surface of a dielectric board 101,for example, as shown in FIG. 1. In the BPF 100, a high-frequency signalis inputted from the first outer conductor pattern 102 a, and ahigh-frequency signal of a predetermined frequency band is selected bythe second to fourth conductor patterns 102 b to 102 d arranged on theinner side and outputted from the fifth outer conductor pattern 102 e.Except for the conductor pattern 102 c at the central part, theconductor patterns 102 are coupled on the lateral side of the board 101.Although not shown, a ground pattern is formed on the entire rear sideof the board 101.

In the BPF 100, the conductor patterns 102 a to 102 e adjacent to eachother cascaded on the major surface of the dielectric board 101 asdescribed above in such a manner that they overlap each other within arange of length of ¼ of the passing wavelength λ, as shown in FIG. 1.Since the conductor patterns 102 are formed on the board 101 having ahigh dielectric constant, the length of each conductor pattern 102 canbe reduced by the wavelength shortening effect of the microstrip lineand the BPF 100 can be miniaturized.

The shortening of wavelength occurs at λ₀/{square root}εw (where λ₀represents the wavelength in vacuum, and εw represents the effectiverelative dielectric constant, which is determined by electromagneticfield distribution of air and dielectric material) on the outer layer ofthe board 101, and also occurs at λ₀/{square root}εr (where εrrepresents the relative dielectric constant of the board). Therefore,the BPF 100 selectively transmits a high-frequency signal of a desiredfrequency band by optimizing the conductor patterns 102 a to 102 e. Inthe BPF 100, since the conductor patterns 102 can be formed by on themajor surface of the board 101 by performing printing or lithographyprocessing as in a general wiring board forming process, these can beformed simultaneously with circuit patterns.

Even in such a BPF 100, the length of each of the conductor patterns 102a to 102 e is regulated by the passing wavelength λ because theconductor patterns 102 a to 102 e overlap each other with an overlappinglength substantially equal to λ/4 of the passing wavelength as they arearrayed. Therefore, the board 101 of a certain size is necessary tocover the lengths of the conductor patterns 102 a to 102 e, and theminiaturization of the BPF 100 is limited.

Meanwhile, another conventional BPF 110 shown in FIGS. 2A to 2C and FIG.3 is formed by a so-called triplate structure in which resonatorconductor patterns 113, 114 are formed within a multilayer boardincluding a pair of dielectric boards 111, 112. Ground patterns 115, 116are formed on the surfaces of the dielectric boards 111, 112,respectively, as shown in FIGS. 2A and 2C. Multiple via-holes 117 areformed in outer circumferential parts of the dielectric boards 111, 112and continuity between the ground patterns 115, 116 on both sides ismade, thus shielding the inner layer circuit.

Each of the resonator conductor patterns 113, 114 has a length M, whichis substantially ¼ of the passing wavelength λ, and the resonatorconductor patterns 113, 114 are formed in parallel with their one endsconnected to the ground patterns 115, 116 and their other ends opened,as shown in FIG. 2B. On the resonator conductor patterns 113, 114,input/output patterns 118, 119 protruding in an arm-like shape towardthe lateral side are formed. In the BPF 110, the resonator conductorpatterns 113, 114 formed in the above-described dielectric boards 111,112 are constructed to have parallel resonance circuits that arecapacitive-coupled like equivalent circuits as shown in FIG. 3.Specifically, in the BPF 110, a parallel resonance circuit PR1 formed bya capacitor C1 and an inductor L1 connected between the resonatorconductor pattern 113 and the ground pattern, and a parallel resonancecircuit PR2 formed by a capacitor C2 and an inductor L2 connectedbetween the resonator conductor pattern 114 and the ground pattern, arecapacitive-coupled via a capacitor C3.

Such a BPF 110 has a function of resonating an open line ofsubstantially λ/2 with respect to a high-frequency signal having awavelength λ, in a predetermined frequency band, and utilizes the facethat the degree of coupling reaches the maximum at λ/4. With this BPF110, a high-frequency signal having a wavelength λ inputted from theresonator conductor pattern 113 is caused to resonate in the bans of thepredetermined passing wavelength λ by the parallel resonance circuit PR1and the parallel resonance circuit PR2. High-frequency components out ofthe band are removed and the signal is then outputted. The BPF 110 isminiaturized as the lengths of the resonator conductor patterns 113, 114formed in the dielectric boards 111, 112 are substantially λ/4.

Meanwhile, as the size and weight of mobile communication devices arefurther reduced, a radio communication module having an overall size of,for example, 10×10 mm or less, is demanded. Particularly in the case ofcarrying a radio communication module on a consumer mobile communicationdevice or the like that has extremely tight cost requirements, the radiocommunication module must be equivalent to an inexpensive printed boardthat is generally used as board material.

The BPF 110 cannot meet the above-described specification requirementsthough the overall length of the resonator conductor patterns 113, 114is reduced to λ/4. That is, in a radio LAN system or a short-distanceradio transmission system called Bluetooth, the carrier frequency bandis regulated to 2.4 GHz and the carrier wavelength λ₀/4 in the space isapproximately 30 mm. Even if the resonator conductor patterns 113, 114are built in a copper-clad multilayer board of FR grade 4 having arelative dielectric constant of approximately 4, which is carried on aradio communication module of a mobile communication device conformableto such a system and is generally used as a board material, for example,a copper-clad multilayer board made of burning-resistant glass clothbase epoxy resin, the passing wavelength λ/4 is approximately 15 mm.Therefore, the BPF 110 cannot meet the above-described specificationrequirements.

It may be considered that, for example, a ceramic material having arelative dielectric constant of 10 or more is used to improve thewavelength shortening effect and thus to miniaturize the BPF 110. Such aBPF 110 needs a large board when integrating peripheral components for aradio communication module, and the cost is increased by the use of theceramic board, which is relatively expensive. Therefore, theabove-described cost requirement cannot be met.

In the above-described BPF 110, the filter characteristics such aspassing band characteristic and cutoff characteristic are determined byelectromagnetic field distribution between the dielectric boards 111,112 and between the resonator conductor patterns 113, 114. In the BPF110, the strength of the electric field changes in accordance with thefacing spacing p between the resonator conductor patterns 113, 114 in anodd excitation mode and also changes in accordance with the spacingbetween the dielectric boards 111, 112 and the resonator conductorpatterns 113, 114 in an even excitation mode, that is, the thickness tof the dielectric boards 111, 112 shown in FIG. 2A. In the BPF 110, thestrength of the electric field also changed in accordance with the widthw of the resonator conductor patterns 113, 114 as shown in FIG. 2A.

In the BPF 110, since the strength of the electric field changes inaccordance with the odd excitation mode or even excitation mode, thedegree of coupling of the resonator conductor patterns 113, 114 changesand the filter characteristics change. In the BPF 110, the dielectricboards 111, 112 and the resonator conductor patterns 113, 114 areprecisely formed in order to realize desired filter characteristics.

Generally, in the BPFs, desired filter characteristics cannot beachieved because of some difference in the manufacturing process, and anadjustment process is performed, for example, based on additionalprocessing for properly changing the position and area of the resonatorconductor patterns while checking their output characteristics by ameasuring device or the like. In the BPF 110, since the resonatorconductor patterns 113, 114 are formed in the inner layer of thedielectric boards 111, 112 as described above, it is difficult toperform such an adjustment process. Therefore, as a highly accuratemanufacturing process to produce each part is employed for the BPF 110,the manufacturing efficiency is lowered and also the yield is lowered.

DISCLOSURE OF THE INVENTION

It is an object of this invention to provide a new filter circuit thatcan solve the problems of the conventional filter circuits as describedabove.

It is another object of this invention to provide a filter circuit thatis miniaturized by acquiring predetermined filter characteristics whilefurther reducing the length of each conductor pattern formed on adielectric board to form a resonator pattern, to less than λ/4 withrespect to a passing wavelength λ.

A filter circuit according to this invention includes a dielectricboard, first to third conductor patterns formed with a length shorterthan λ/4 of a passing wavelength λ as distributed line patterns parallelto each other in the dielectric board, and a first capacitor and asecond capacitor. The first conductor pattern has its one end groundedand has its other end opened, and a high-frequency signal is inputted tothe first conductor pattern. The second conductor pattern has its oneend grounded and has its other end opened, and it outputs ahigh-frequency signal of a predetermined frequency band selected frominputted high-frequency signals. The third conductor pattern has itsboth ends opened. The first capacitor and the second capacitor addparallel capacitance based on a concentrated constant to the firstconductor pattern and the second conductor pattern.

The filter circuit according to this invention has a third capacitor foradding serial capacitance based on a lumped constant to the firstconductor pattern and the second conductor pattern and thus making afrequency notch effect. Moreover, in the filter circuit, a capacitor forcapacitance adjustment is connected to the first capacitor and thesecond capacitor via switching means.

In the filter circuit according to this invention, the first to thirdconductor patterns are electromagnetically coupled and resonate in apredetermined frequency band corresponding to the passing frequency λ,and a high-frequency signal of a predetermined frequency band selectedfrom high-frequency signals inputted to the first conductor pattern isoutputted from the second conductor pattern. In this filter circuit,inductive electromagnetic coupling is made between the first conductorpattern and the second conductor pattern, each of which is formed withthe length shorter than λ/4 of the passing wavelength λ and has itsdistal end short-circuited, and capacitive electromagnetic coupling ismade between the first conductor pattern and the second conductorpattern, and the third conductor pattern, which has its distal endopened. In the filter circuit according to this invention, as theinternal capacitance formed by each conductor pattern and the parallelcapacitance added by the first capacitor and the second capacitor areoptimally set, the resonance frequency band prescribed by the lengths ofthe first conductor pattern and the second conductor pattern is lowered,and predetermined filter characteristics are maintained andminiaturization is realized even when each conductor pattern is formedwith a length much shorter than λ/4.

The other objects of this invention and specific advantages provided bythis invention will be further clarified by the following description ofembodiments described with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view showing a conventional band-pass filter.

FIGS. 2A to 2C show a conventional band-pass filter of a triplatestructure. FIG. 2A is a sectional view thereof. FIG. 2B is a plan viewshowing a dielectric board on which resonator conductor patterns areformed. FIG. 2C is a plan view showing the dielectric board on which aground pattern is formed.

FIG. 3 is a circuit diagram showing parallel resonance circuits of theconventional band-pass filter of the triplate structure.

FIG. 4 is a schematic plan view showing the structure of a band-passfilter according to this invention.

FIG. 5 is a graph showing the length of line and the passing frequencyrelated to an electromagnetic coupling operation of a pair of linepatterns in a transmission circuit.

FIG. 6 is a circuit diagram showing a parallel resonance circuit of theband-pass filter.

FIG. 7 is a schematic longitudinal sectional view in the direction ofwidth showing the structure of each conductor pattern built in adielectric board of the band-pass filter.

FIG. 8 is a longitudinal sectional view thereof in the direction oflength.

FIG. 9 is a schematic longitudinal sectional view of a communicationmodule board equipped with the band-pass filter.

FIG. 10 a schematic plan view of another band-pass filter having astructure for adjusting parallel capacitance to be added to a firstconductor pattern and a second conductor pattern.

FIG. 11 is a schematic plan view of another band-pass filter havingparallel capacitance adjustment structure using MEMS switches.

FIG. 12A is a longitudinal sectional view of a MEMS switch in anon-continuity state. FIG. 12B is a schematic longitudinal sectionalview of the MEMS switch in an operating state.

FIG. 13 is a circuit diagram showing a band-pass filter circuit having aband-pass filter equipped with MEMS switch to form feedback logic.

FIG. 14 is a schematic longitudinal sectional view showing the band-passfilter.

FIG. 15 is a graph showing filter characteristics of the band-passfilter.

FIG. 16 is a schematic longitudinal sectional view showing a band-passfilter having conductor patterns formed on the surface of a dielectriclayer.

FIG. 17 is a schematic longitudinal sectional view showing a band-passfilter having conductor patterns formed on the surface of a dielectriclayer and having a shield cover provided over them.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an exemplary application of this invention to a band-passfilter (BPF) based on distributed constant design will be described. ABPF is used, for example, for a band-pass filter circuit forming anantenna input/output unit of a communication function module unit,though not shown. It has a characteristics of passing atransmitted/received signal superimposed on a 2.4 GHz carrier frequency,for example, in accordance with the radio LAN system, Bluetooth or thelike, transmitted and received by an antenna. A BPF 1 has a triplatestructure having first to third conductor patterns 8 to 10, an inputconductor pattern 11 and an output conductor pattern 12, which will bedescribed later in detail, patterned within a dielectric board 2, asshown in FIG. 4.

The BPF 1 has the dielectric board 2 including a base board 3 and aresin board stacked on the base board 3, as shown in FIG. 7. As the baseboard 3, for example, a copper-clad multilayer board of FR grade 4having a copper foil layer formed on one major surface of a glass epoxyboard is used. The resin board 4 is formed by stacking dielectricinsulating layers 6, 7 having a predetermined thickness on both sides ofa core 5. The first to third conductor patterns 8 to 10, which will bedescribed later in detail, are patterned on a major surface of thedielectric insulating layer 6 forming the stacking surface to the baseboard 3, and a ground pattern is formed on a major surface of thedielectric insulating layer 7. The dielectric board 2 thus has theabove-described triplate structure.

In the dielectric board 2, each of the dielectric insulating layers 6, 7on the resin board 4 is made of a dielectric insulating material havinga predetermined thickness and characteristics of low dielectric constantand low Tanδ, that is, excellent high-frequency characteristics.Specifically, the dielectric insulating layers 6, 7 are made of anorganic dielectric resin material such as polyphenylethylene (PPE),bismaleidetriazine (BT-resin), polytetrafluoroethylene (Teflon:trademark registered), polyimide, liquid crystal polymer, polynorbornene(PNB) or polyolefin resin, an inorganic dielectric material such asceramics, or a mixture of an organic dielectric resin material and aninorganic dielectric material. Also for the base board 3, a similardielectric insulating material may form a base material.

In the BPF 1, via-holes 13 are suitably formed in the base board 3 andthe resin board 4 of the dielectric board 2, as shown in FIGS. 7 and 8.Through these via-holes 13, wiring patterns 15 formed on an inner layerare connected to a metal layer 14 on the base board 3. The metal layer14 is formed substantially on the entire major surface of the base board3 and serves as a ground pattern 14. Interlayer connection is madebetween the ground pattern 14 and the ground pattern on the side of thedielectric insulating layer 7 through the via-holes 13 in the outercircumferential part of the dielectric board 2.

The BPF 1 has a first capacitor 16 and a second capacitor 17 that areconnected in parallel to the first conductor pattern 8 and the secondconductor pattern 9 via a first short-circuit pattern 15 a and a secondshort-circuit pattern 15 b, as shown in FIG. 4. The BPF 1 has a thirdcapacitor 18 connected in series to the first conductor pattern 8 andthe second conductor pattern 9 via a wiring pattern 15 c. In the BPF 1,for example, the first capacitor 16 and the second capacitor 17 areformed as film-forming elements in the dielectric insulating layer 6 orthe dielectric insulating layer 7, and the third capacitor 18 is mountedas a chip component connected via the via-hole 13 on the major surfaceof the dielectric insulating layer 7.

The first conductor pattern 8 and the second conductor pattern 9 areformed by relatively wide rectangular patterns and are made parallel toeach other to face each other at a predetermined spacing in thelongitudinal direction, as shown in FIG. 4. The third conductor pattern10 is formed by a narrow rectangular pattern, situated between the firstconductor pattern 8 and the second conductor pattern 9, and madeparallel to these conductor patterns over the entire length. These firstto third conductor patterns 8 to 10, the input conductor pattern 11 andthe output conductor pattern 12 are patterned by a conventionally usedtechnique including, for example, a metal foil attaching process, apatterning process by photolithography, or an etching process.

The input conductor pattern 11 is protruding in an arm-like shape fromthe first conductor pattern 8, thus forming a conductor pattern on theprimary side where a high-frequency signal is inputted. As shown in FIG.4, one end side of the first conductor pattern 8 is a short-circuit end8 a connected to the ground pattern 14 through the via-hole 13, and theother end side is an open end 8 b. Similarly, the output conductorpattern 12 is protruding in an arm-like shape from the second conductorpattern 9, thus forming a conductor pattern on the secondary side wherea high-frequency signal of a predetermined frequency band selected frominputted high-frequency signals is outputted, as will be described laterin detail. Again, one end side of the second conductor pattern 9 is ashort-circuit end 9 a connected to the ground pattern 14 through thevia-hole 13, and the other end side is an open end 9 b.

The first conductor pattern 8 and the second conductor pattern 9 havethe same length. This length N is N<<λ/4, which is means that N is muchshorter than the electric length λ/4 of approximately 6 mm with respectto the passing wavelength λ of the carrier frequency band. The firstconductor pattern 8 and the second conductor pattern 9 are formed with alength of approximately 2.7 mm, while the electric length λ/4 withrespect to the passing wavelength λ of the 2.4 GHz carrier frequencyband is approximately 6 mm. Also the third conductor pattern 10 has alength of approximately 2.7 mm, which is the same length as the lengthof the first conductor pattern 8 and the second conductor pattern 9.

Meanwhile, in transmission lines, a distal end short-circuit type lineand a distal end open type line of a pair of electromagnetically coupledlines show difference operation characteristics, that is, inductiveoperation characteristic and capacitive operation characteristic, inaccordance with a line length k with respect to the passing wavelengthλ, as shown in FIG. 5. Specifically, the distal end short-circuit typeline shows an inductive operation characteristic (inductor) within arange of 0<k<λ/4, as indicated by a solid line A in FIG. 5. On the otherhand, the distal end open type line shows a capacitive operationcharacteristic (capacitor) within a range of 0<k<λ/4, as indicated by adotted line in FIG. 5.

The BPF 1 according to this invention has the basic structure in whichthe first to third conductor patterns 8 to 10 formed in the dielectricboard 2 utilize the resonance characteristics prescribed by theirrespective lengths, as in the above-described conventional BPF 110.However, the BPF 1 has the structure including inductive elements and acapacitive element. Specifically, in the BPF 1, the first conductorpattern 8 and the second conductor pattern 9, which have theabove-described length and have their respective one endsshort-circuited, are electromagnetically coupled to form an inductor LIand an inductor LO, respectively. In the BPF 1, the third conductorpattern 10, which has the above-described length and has its both endsopened, form a capacitor C3 with respect to the first conductor pattern8 and the second conductor pattern 9.

In the BPF 1, the first to third conductor patterns 8 to 10, the firstcapacitor 16 and the second capacitor 17 form an equivalent circuit asshown in FIG. 6. Specifically, in the BPF 1, the primary side inductorLI formed by the first conductor pattern 8 and the ground pattern 14,and the secondary side inductor LO formed by the second conductorpattern 9 and the ground pattern 14 are electromagnetically coupled. Inthe BPF 1, these primary side inductor LI and secondary side inductor LOare capacitive-coupled via the capacitor C3 formed by the thirdconductor pattern 10 and the ground pattern 14.

Moreover, in the BPF 1, parallel capacitance is added to the primaryside inductor LI by the first capacitor 16, and parallel capacitance isadded to the secondary side inductor LO by the second capacitor 17. Inthe BPF 1, the third capacitor 18 is connected in series between thefirst capacitor 16 and the second capacitor 17, thus adding serialcapacitance to the primary side inductor LI and the secondary sideinductor LO.

In the BPF 1 according to this invention, since the first conductorpattern 8 and the second conductor pattern 9 are formed with a lengthmuch shorter than λ/4 with respect to the wavelength λ of the inputtedhigh-frequency signal, as described above, resonance is generated in afrequency band higher than the desired passing wavelength λ by theelectromagnetically coupled primary side inductor LI and secondary sideinductor LO. Meanwhile, in the BPF 1, since parallel capacitance isadded to the primary side inductor LI and the secondary side inductor LOby the first capacitor 16 and the second capacitor 17, the resonancefrequency band raised by the shortening of the pattern length is loweredand the degree of coupling is maximized similarly to the line length ofλ/4. Therefore, with the BPF 1, a high-frequency signal having thewavelength λ inputted from the side of the first conductor pattern 8resonates in the band of the predetermined passing wavelength λ so thatthe high-frequency components out of the band are removed, and theresulting signal is outputted from the side of the second conductorpattern 9.

In the BPF 1, the frequency notch effect on the inputted high-frequencysignal is performed by the third capacitor 18 inserted in series betweenthe first capacitor 16 and the second capacitor 17. Therefore, with theBPF 1, trap and attenuation pole components are reduced and ahigh-frequency signal from which unwanted components have been removedis outputted from the second conductor pattern 9 in a stable condition.

The BPF 1 constructed as described above may include a communicationmodule board 20, for example, as shown in FIG. 9. The communicationmodule board 20 includes a base board part 21 made of an organic boardhaving multiple wiring layers formed thereon and having the uppermostlayer flattened, and a high-frequency circuit part 22 stacked on thebase board part 21. In the communication module board 20, though notdescribed in detail, a power circuit and a control circuit are formed inthe base board part 21, and the BPF 1 and a high-frequency signalcircuit or processing circuit are formed in the high-frequency circuitpart 22.

In the communication module board 20, a sufficiently large area forforming the power circuit and ground can be provided on the base boardpart 21, and power supply with high regulation is carried out. In thecommunication module board 20, since electrical isolation from thehigh-frequency circuit part 22 is made and occurrence of interferenceare restrained, its properties are improved.

In the communication module board 20, a relatively inexpensive organicboard is used as the base, and an insulating dielectric layer 23 made ofthe above-described insulating dielectric material is stacked on theflattened uppermost layer, thus forming the high-frequency circuit part22. In the communication module board 20, a suitable wiring pattern 24and a passive element 25 such as an inductor element, capacitor elementor resistor element are formed by a thin film forming technique in theinsulating dielectric layer 23. In the communication module board 20, achip component 26 is mounted on the high-frequency circuit part 22, asshown in FIG. 9.

In the BPF manufacturing process, generally, since predetermined filtercharacteristics cannot be acquired in some cases because of thedifference during the manufacturing process, processing to adjust theposition and shape of each part is performed while checking the outputcharacteristics by a measuring device or the like. However, in the BPF1, it is difficult to perform such adjustment processing since the firstto third conductor patterns 8 to 10, the first capacitor 16 and thesecond capacitor 17 are formed within the dielectric board 2, asdescribed above.

In a BPF 30 shown in FIG. 10, a first capacitor 31 and a secondcapacitor 32 for capacitance adjustment are connected in parallel to thefirst capacitor 16 and the second capacitor 17 for adding parallelcapacitance to the first conductor pattern 8 and the second conductorpattern 9, respectively. The first capacitor 31 and the second capacitor32 are mounted on the surface of the dielectric board 2, for example, aschip components, and are connected to the first capacitor 16 and thesecond capacitor 17 via the via-holes 13.

The BPF 30 is adjusted to achieve desired output characteristics bysuitably replacing the first capacitor 31 and the second capacitor 32,which are made of mounting-type chip components. Of course, in the BPF30, it is possible to use capacitors made of chip components instead ofthe above-described built-in type first capacitor 16 and secondcapacitor 17. However, chip capacitors have such a characteristic thatas the capacitance value increase, the self-resonance frequency islowered and the capacitance value jumps more roughly. In the BPF 30, asthe built-in type first capacitor 16 and second capacitor 17, and thechip-type first capacitor 31 and second capacitor 32 having a smallcapacitance value, are connected in parallel, fine tuning of ahigh-frequency signal is accurately carried out.

A later adjustment process can be carried out also in a BPF 35 shown inFIG. 1. The BPF 35 has plural first capacitance adding circuits formedby series circuits including first MEMS switches 36 a to 36 n and firstcapacitors 37 a to 37 n and connected to the first conductor pattern 8via an array pattern 15 d, and plural second capacitance adding circuitsformed by series circuits including second MEMS switches 38 a to 38 nand second capacitors 39 a to 39 n and connected to the second conductorpattern 9 via an array pattern 15 e.

In the BPF 35 shown in FIG. 11, as the first MEMS switches 36 a to 36 nare selectively switched, the connection state between the firstconductor pattern 8 and the group of first capacitors 37 is switched toadjust the added capacitance. Similarly, as the second MEMS switches 38a to 38 n are selectively switched, the connection state between thesecond conductor pattern 9 and the group of second capacitors 39 a to 39n is switched to adjust the added capacitance.

FIGS. 12A and 12B show a typical MEMS (micro electromechanical system)switch 40. The MEMS switch 40 is entirely covered with an insulatingcover 41, as shown in FIG. 12A. In the MEMS switch 40, a first fixedcontact 43, a second fixed contact 44 and a third fixed contact 45 areformed on a silicon substrate 42 and insulated from each other. In theMEMS switch 40, a flexible moving contact piece 46 of a thin plate shapeis rotatably supported at its one side on the first fixed contact 43. Inthe MEMS switch 40, the first fixed contact 43 and the third fixedcontact 45 are used as input/output contacts and connected toinput/output terminals 48 a, 48 b provided on the insulating cover 41via leads 47 a, 47 b, respectively.

In the MEMS switch 40, one end of the moving contact piece 46 is aconstantly closed contact to the first fixed contact 43 on the side ofthe silicon substrate 42, and its freer end forms a constantly opencontact to the third fixed contact 45. An electrode 49 is providedwithin the moving contact piece 46, corresponding to the second fixedcontact 44 formed at the central part. In the MEMS switch 40, in thenormal state, one end of the moving contact piece 46 is in contact withthe first fixed contact 43 and its other end is held in a non-contactstate with the third fixed contact 45, as shown in FIG. 12A.

Each MEMS switch 40 constructed as described above is mounted on themajor surface of the dielectric board 2. One input/output terminal 48 aof each MEMS switch 40 is connected to the array patterns 15 d, 15 e andthe other input/output terminal 48 b is connected to the firstcapacitors 37 or the second capacitors 39. Therefore, the MEMS switch 40maintains the insulating state of the array patterns 15 d, 15 e, thatis, between the first conductor pattern 8 and the first capacitors 37 orbetween the second conductor pattern 9 and the second capacitors 39.

When a driving signal is inputted to the MEMS switch 40, a drivingvoltage is applied to the second fixed contact 44 and the internalelectrode 49 of the moving contact piece 46. In the MEMS switch 40, thisgenerates an attracting force between the second fixed contact and themoving contact piece 46, and the moving contact piece 46 is displacedabout the first fixed contact 43 as the fulcrum toward the siliconsubstrate 42 and has its free end connected to the third fixed contact45. This connection state is maintained. In the MEMS switch 40, when adriving voltage of backward bias is applied to the second fixed contact44 and the internal electrode 49 of the moving contact piece 46 in theabove-described state, the moving contact piece 46 restores its initialstate and the connection state with the third fixed contact 45 iscanceled. Since the MEMS switch 40 is a switch that is very small andneeds no holding current for holding the operating state, providing theMEMS switch 40 in the BPF 35 does not increase the size of the BPF 35and also realizes lower power consumption.

In the BPF 35, as a reference signal is inputted to the input conductorpattern 11 on the side of the first conductor pattern 8 and on/offcontrol of the first MEMS switches 36 and the second MEMS switches 38 iscarried out while measuring an output from the output conductor pattern12 on the side of the second conductor pattern 9, the filtercharacteristics are adjusted. Therefore, the BPF 35 forms feedback logicof a band-pass filter circuit, for example, as shown in FIG. 13. Theband-pass filter circuit is given a characteristic of passing ahigh-frequency signal superimposed on a 2.4 GHz frequency band, andincludes a BPF 51, an amplifier 52, a mixer 53 and an oscillator 54,which process a signal received by an antenna 50. In the band-passfilter circuit, a second BPF 55 passes a high-frequency signal of apredetermined frequency band outputted from mixer 53 and supplies thesignal to a receiving amplifier 56.

In the band-pass filter circuit, in consideration of the filtercharacteristics prescribed by the thickness of the dielectric board 2and the position, shape and the like of the first to third conductorpatterns 8 to 10, when a certain change occurs in the environment of thedevice in which the band-pass filter circuit is used, for example, whena metallic material or dielectric material is arranged closely to thedevice or the temperature or humidity changes, the frequencycharacteristics of the BPF 51 may be deviated and the received powerfrom the antenna 50 may be lowered. In the band-pass filter circuit, theoutput level of the receiving amplifier 56 is detected, and when alowering state is detected, the detection output is sent to a switchdriving circuit part 57.

In the band-pass filter circuit, a control signal S for driving thefirst MEMS switches 36 and the second MEMS switches 38 is generated bythe switch driving circuit part 57 and is fed back to the BPF 51. In theband-pass filter circuit, as on/off control of the first MEMS switches36 and the second MEMS switches 38 is selectively carried out, thefrequency characteristics are fine-tuned as described above.

The capacitance adjustment structure is not limited to theabove-described structure of the BPF 35. For example, instead of thefirst MEMS switches 36 and the second MEMS switches 38, an open statemay be provided between the array patterns 15 d, 15 e and the first andsecond capacitors 37, 39, and conductive paste such as silver paste or acopper foil may be suitably attached later to form a short circuit.

With respect to the BPF according to this invention constructed asdescribed above, FIG. 15 shows the result of property simulation basedon the specifications of a BPF 60 shown in FIG. 14. In the BPF 60, firstto third conductor patterns 62 to 64 of the above-described structureare patterned in a dielectric layer 61, and first to third capacitorsare provided, though not shown. The BPF 60 has a triplate structure inwhich ground patterns 65, 66 are formed on both sides of the dielectriclayer 61. In the BPF 60, a thin film layer 67 is stacked on the groundpattern 66.

In the BPF 60, the dielectric layer 61 has a total thickness ofapproximately 0.7 mm and an average relative dielectric constant of 3.8.In the BPF 60, the first conductor pattern 62 and the second conductorpattern 63 are formed with a length of approximately 2.7 mm, and thefirst capacitor and the second capacitor for adding parallel capacitanceto the first conductor pattern 62 and the second conductor pattern 63have capacitance of approximately 3 pF each. In the BPF 60, the thirdcapacitor for adding serial capacitance has capacitance of approximately0.7 pF. Of course, in the BPF 60, the first conductor pattern 62 and thesecond conductor pattern 63 have their respective one endsshort-circuited and the third conductor pattern 64 has its both endsopened.

In the BPF 60, the first conductor pattern 62 and the second conductorpattern 63 are formed with a length much shorter than λ/4 of the passingwavelength λ, as described above. However, as can be seen from FIG. 15,the maximum resonance characteristic appears in the 2.4 GHz band withoutbeing prescribed by the lengths of the first conductor pattern 62 andthe second conductor pattern 63.

While the first to third conductor patterns 8 to 10 are patterned on theinner layer of the dielectric board 2 in the above-describedembodiments, this invention is not limited to this structure. In a BPF70 shown in FIG. 16, first to third conductor patterns 72 to 74 arepatterned on a major surface of a dielectric layer 71. In the BPF 70, aground pattern 75 is formed entirely over the other major surface of thedielectric layer 71, and a thin film layer 76 is formed on the groundpattern 75. In the BPF 70, the first to third conductor patterns 72 to74 form a microstrip line structure.

In a BPF 80 shown in FIG. 17, a shield case 81 is combined with thedielectric layer 71 of the above-described BPF 70. In the BPF 80, thefirst to third conductor patterns 72 to 74 are enclosed by thedielectric layer 71 and a dielectric layer of air between the groundpattern 75 and the shield case 81, thus forming a strip line structure.In the BPF 80, loss due to parasitic capacitance is reduced by theshield case 81.

It should be understood by those ordinarily skilled in the art that theinvention is not limited to the embodiments illustrated in theaccompanying drawings and described in the above description in detail,but various modifications, alternative constructions or equivalents canbe implemented without departing from the scope and spirit of thepresent invention as set forth and defined by the appended claims.

INDUSTRIAL APPLICABILITY

The filter circuit according to this invention has first to thirdconductor patterns that are formed as distributed line patterns parallelto each other on a dielectric board and electromagnetically coupled witheach other. A first capacitor and a second capacitor add parallelcapacitance to the first conductor pattern and the second conductorpattern, which have their distal ends short-circuited for inductivecoupling, and these conductor patterns are capacitive-coupled with thethird conductor pattern, which is formed by an open pattern, thusforming an internal capacitor. Therefore, while the first to thirdconductor patterns are formed with a length much shorter than λ/4 of thepassing wavelength, the resonance frequency band can be lowered by thecombination of internal capacitance and parallel capacitance to beadded, irrespective of the line length of each conductor pattern. Thus,miniaturization is realized and desired frequency characteristics can beacquired.

Moreover, in the filter circuit according to this invention, as thecapacitance of the first capacitor and the second capacitor is adjusted,an optimum filter characteristic value can be set even when the filtercharacteristics are varied or deviated because of some difference duringthe manufacturing process or changes in the environment. This improvesthe productivity and yield of the filter circuit and also improves thereliability and performance.

1. A filter circuit characterized by comprising: a dielectric board; afirst conductor pattern formed as a distributed line pattern in thedielectric board and having one end grounded and the other end opened,the first conductor pattern having high-frequency signals inputtedthereto; a second conductor pattern formed as a distributed line patternparallel to the first conductor pattern in the dielectric board andhaving one end grounded and the other end opened, the second conductorpattern being electromagnetically coupled with the first conductorpattern and thus outputting a high-frequency signal of a predeterminedfrequency band selected from the high-frequency signals inputted to thefirst conductor pattern; a third conductor pattern formed as adistributed line pattern parallel to the first conductor pattern and thesecond conductor pattern in the dielectric board and having both endsopened; and a first capacitor and a second capacitor for adding parallelcapacitance based on a concentrated constant to the first conductorpattern and the second conductor pattern; wherein as each of the firstto third conductor patterns is formed with a length shorter than λ/4with respect to a passing wavelength λ, inductive electromagneticcoupling is carried out between the first conductor pattern and thesecond conductor pattern, and capacitive electromagnetic coupling iscarried out between the first and second conductor patterns and thethird conductor pattern.
 2. The filter circuit as claimed in claim 1,characterized by comprising a third capacitor for adding serialcapacitance based on a lumped constant to the first conductor patternand the second conductor pattern.
 3. The filter circuit as claimed inclaim 2, characterized in that the first to third capacitors arecapacitor elements formed as thin films in the dielectric board,capacitor chip elements mounted on the dielectric board, or acombination of capacitor elements formed as thin films in the dielectricboard and capacitor chip elements mounted on the dielectric board. 4.The filter circuit as claimed in claim 1, characterized in that acapacitor for capacitance adjustment is connected to the first capacitorand the second capacitor via switching means.
 5. The filter circuit asclaimed in claim 1, characterized in that, on an inner layer of thedielectric board, the first to third conductor patterns are formed andthe first capacitor and the second capacitor are formed as thin films,plural capacitance adjusting circuits, each including switching meansand a capacitance adjustment capacitor and connected parallel to thefirst capacitor or the second capacitor through a via-hole, are providedon an outer layer of the dielectric board, and each of the switchingmeans is switched to adjust parallel capacitance to be added to thefirst capacitor or the second capacitor by each of the capacitanceadjustment capacitors.
 6. The filter circuit as claimed in claim 1,characterized in that the first to third capacitors are formed on afirst outer layer of the dielectric board, and a metal plate forcovering and shielding the first outer layer is provided on thedielectric board and a ground pattern is formed on a second outer layer,so that the first to third conductor patterns form a strip linestructure.
 7. The filter circuit as claimed in claim 1, characterized inthat the dielectric board is formed by a buildup layer including adielectric insulating layer and a wiring pattern stacked on a buildupforming surface of a base board part in which multiple wiring layers areformed on a base board made of an organic board and an uppermost layeris flattened to form the buildup forming surface, and in the builduplayer, the first to third conductor patterns are patterned and the firstcapacitor and the second capacitor are formed as thin films.